1. Field of the Invention
The present invention relates generally to mercaptan terminated polymers. More particularly, the present invention relates to synthesis of low viscosity polymeric polythiols and their use in compositions (and methods) for decreasing the viscosity of polymer formulations without the use of plasticizers or volatile organic components which decrease the viscosity at a cost to the final properties and are environmentally unacceptable.
2. Description of Related Art
Mercaptan terminated polymers are widely used in sealant, adhesive and coating compositions where quick or low temperature cures are needed. The polythioethers and polysulfides among this group also have good fuel, solvent, water and temperature resistance. Exemplary polymers within this group are described in U.S. Pat. Nos. 3,923,748; 4,355,307; 3,640,965; 4,425,389; 4,689,395 and 4,609,762.
The sealants, adhesives and coatings which contain these mercaptan terminated polymers are applied to surfaces as flowable liquids. In general, mercaptan terminated polymers have relatively high viscosities which make them difficult to formulate into viable finished products. One method for reducing the viscosity of the polymer compositions involves adding various volatile organic solvents to these compositions. Although the use of volatile diluents provides sufficient reduction in polymer viscosity, which results in easy to apply compositions, there are a number of drawbacks associated with the use of such volatile components. For example, the volatile diluents evaporate during the curing process and cause shrinking of sealant, adhesive or coating. In addition, the evaporating volatile components must be contained and dealt with in accordance with increasingly strict environmental regulations.
Another approach for providing flowable mercaptan terminated polymer sealants involves the use of plasticizers. Plasticizers eliminate shrinking problems because they do not evaporate from the polymer during the curing process. However, the plasticizer which remains in the cured sealant causes the sealant to have lower hardness and lower modulus. In many cases limited compatability of plasticizer and polymer results in slow phase separation leading to cracks in the applied materials. Blending of plasticizers also results in dirt collection at the surface of the applied compositions.
The problems associated with the use of volatile diluents and plasticizers become even more evident when compositions having increased hardness are desired. Increased hardnesses are typically achieved by adding extra filler to the formulation or adding hard aromatic segments to the polymer backbone. The use of extra filler or hard aromatic segments increases the viscosity resulting in decreased workability. To offset the increase in viscosity, extra organic diluent or plasticizers must be added. Additional filler may then have to be added to account for the extra diluent or plasticizers. This creates problems because excessive use of fillers decreases the elastomeric character of the final product.
It would be desirable to provide compositions and methods which do not rely on volatile organic diluents or plasticizers to lower viscosities. Such compositions and methods must be able to reduce the viscosity to a level which allows good workability without adversely affecting other desirable characteristics. For example, characteristics such as working time, curing time and long term stability should not be adversely affected by the compositions and methods used to reduce viscosity.